Connected: An Internet Encyclopedia
RFC 854 - TELNET COMMAND STRUCTURE

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### TELNET COMMAND STRUCTURE

All TELNET commands consist of at least a two byte sequence: the "Interpret as Command" (IAC) escape character followed by the code for the command. The commands dealing with option negotiation are three byte sequences, the third byte being the code for the option referenced. This format was chosen so that as more comprehensive use of the "data space" is made -- by negotiations from the basic NVT, of course -- collisions of data bytes with reserved command values will be minimized, all such collisions requiring the inconvenience, and inefficiency, of "escaping" the data bytes into the stream. With the current set-up, only the IAC need be doubled to be sent as data, and the other 255 codes may be passed transparently.

The following are the defined TELNET commands. Note that these codes and code sequences have the indicated meaning only when immediately preceded by an IAC.

          NAME               CODE              MEANING

          SE                  240    End of subnegotiation parameters.
          NOP                 241    No operation.
          Data Mark           242    The data stream portion of a Synch.
                                     This should always be accompanied
                                     by a TCP Urgent notification.
          Break               243    NVT character BRK.
          Interrupt Process   244    The function IP.
          Abort output        245    The function AO.
          Are You There       246    The function AYT.
          Erase character     247    The function EC.
          Erase Line          248    The function EL.
          Go ahead            249    The GA signal.
          SB                  250    Indicates that what follows is
                                     subnegotiation of the indicated
                                     option.
          WILL (option code)  251    Indicates the desire to begin
                                     performing, or confirmation that
                                     you are now performing, the
                                     indicated option.
          WON'T (option code) 252    Indicates the refusal to perform,
                                     or continue performing, the
                                     indicated option.
          DO (option code)    253    Indicates the request that the
                                     other party perform, or
                                     confirmation that you are expecting
                                     the other party to perform, the
                                     indicated option.
          DON'T (option code) 254    Indicates the demand that the
                                     other party stop performing,
                                     or confirmation that you are no
                                     longer expecting the other party
                                     to perform, the indicated option.
          IAC                 255    Data Byte 255.

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Connected: An Internet Encyclopedia
RFC 854 - TELNET COMMAND STRUCTURE